In the automotive industry, a silver composition is applied to window glass and fired to form electrically conductive silver tracks on the glass. These tracks are used particularly on rear, and less often on front or side, windows to provide a de-mister. Current is usually distributed through individual conductor tracks by means of a wider silver strip, termed a bus bar, at the edges of the window.
In order to protect the adhesive used to glue windows into the motor vehicle body from being degraded by ultra-violet light, and also for aesthetic and other purposes, an enamel layer, usually black or grey, is provided around the edges of the window. It has become common to provide the silver bus bars on the enamel so as to obscure them from view from outside the vehicle. This, however, is not as effective as is desired. On firing the silver composition and enamel composition to form the silver tracks and enamel, silver ions tend to migrate from the silver composition through the enamel composition into the glass-enamel interface where they tend to become visible after reaction with reducing species present in the glass. This is manifested by a dark amber colour, which is particularly noticeable in bright sunlight, or sometimes by a blue colour, particularly when viewed through green or blue glass. The effect is undesired and numerous proposals have been made for overcoming the problem. For instance, it has been proposed to incorporate reducing agents into the enamel composition used to form the enamel, so that on firing, the reducing agents react with the migrating silver ions, reducing them to elemental silver to prevent or impair them from interacting with the glass to form visible tracks--see for instance U.S. Pat. No. 5,141,798. Enamels formed from enamel compositions containing reducing agents, however, can still form a blue colour under the bus bar, and this is particularly evident when green or blue glass is employed; green or blue glass is increasingly being employed to decrease the infra-red transmission of the glass, but the blue colouration is evident also with clear glass at high firing times and temperatures. Large amounts of the reducing agents can be required to obscure the silver tracks, and this can be expensive or lead to disadvantages, for instance undesired colour shades. Incorporating a reducing agent dilutes the effect of the pigment employed in the enamel composition to provide the colour to the enamel layer. The reducing agent can adversely affect the acid resistance of the enamel. A different approach is shown by U.S. Pat. No. 5,250,475 and U.S. Pat. No. 5,350,718. U.S. Pat. No. 5,250,475 discloses black glass paste comprising ground, sulphide or polysulphide modified lead borosilicate glass which has 0.1 to 12% by weight--based on borosilicate glass--of sulphides and/or polysulphides fused therein, 10 to 35% by weight of black ground inorganic pigments and 15 to 25% by weight of a carrier medium. U.S. Pat. No. 5,350,718 discloses a glass frit essentially free of lead for forming an enamel capable of preventing Ag.sup.+ migration, said glass frit comprising 15 to 70% by weight zinc oxide, 15 to 40% by weight silicon dioxide and 5 to 25% by weight boron trioxide and containing dissolved in said glass 0.05 to 15 mol % of at least one member of the group consisting of sulphur and sulphides. The frits of these two specifications are prepared by fusing together the frit components including the sulphur moieties. This process is not as controlled as is desirable. In addition, during the heating, sulphur may not be incorporated or sulphur or hydrogen sulphide may be liberated, eg by reaction of water--hydrogen sulphide is toxic and has a bad smell. The frit containing sulphur moieties also has disadvantages. Its thermal expansion coefficient is not an appropriate match to that of the window glass to which the enamel composition is applied. In addition, the frit tends to cause brown or black silver sulphide stains by reaction of the enamel with the overlying silver.
A need exists for an alternative way of obscuring the silver tracks. An object of the present invention is to meet this need. A need exists for an improved enamel composition, both for this use and for other applications to glass. An object of the present invention is to meet this need also.